The rifaximin (INN; see The Merck Index, XIII Ed., 8304) is an antibiotic pertaining to the rifamycin class, exactly it is a pyrido-imidazo rifamycin described and claimed in the Italian Patent IT 1154655, while the European Patent EP 0161534 describes and claims a process for its production starting from the rifamycin O (The Merck Index, XIII Ed., 8301).
Both these patents describe the purification of the rifaximin in a generic way saying that the crystallization can be carried out in suitable solvents or solvent systems and summarily showing in some examples that the product coming from the reaction can be crystallized from the 7:3 mixture of ethyl alcohol/water and can be dried both under atmospheric pressure and under vacuum without saying in any way neither the experimental conditions of crystallization and drying, nor any distinctive crystallographic characteristic of the obtained product.
The presence of different polymorphs had not been just noticed and therefore the experimental conditions described in both patents had been developed with the goal to get a homogeneous product having a suitable purity from the chemical point of view, apart from the crystallographic aspects of the product itself.
It has now be found, unexpectedly, that some polymorphous forms exist whose formation, in addition to the solvent, depends on the conditions of time and temperature at which both the crystallization and the drying are carried out.
These orderly polymorphous forms will be, later on, conventionally identified as rifaximin δ (FIG. 1) and rifaximin ε (FIG. 2) on the basis of their respective specific diffractograms reported in the present application.
The polymorphous forms of the rifaximin have been characterized through the technique of the powder X-ray diffraction.
The identification and characterization of these polymorphous forms and, contemporarily, the definition of the experimental conditions for obtaining them is very important for a compound endowed with pharmacological activity which, like the rifaximin, is marketed as medicinal preparation, both for human and veterinary use. In fact it is known that the polymorphism of a compound that can be used as active principle contained in a medicinal preparation can influence the pharmaco-toxicologic properties of the drug. Different polymorphous forms of an active principle administered as drug under oral or topical form can modify many properties thereof like bioavailability, solubility, stability, color, compressibility, flowability and workability with consequent modification of the profiles of toxicological safety, clinical effectiveness and productive efficiency.
What above mentioned is confirmed with authority by the fact that the authorities that regulate the grant of the authorization for the admission of the drugs on the market require that the manufacturing methods of the active principles are standardized and controlled in such a way that they give homogeneous and sound results in terms of polymorphism of the production batches (CPMP/QWP/96, 2003—Note for Guidance on Chemistry of new Active Substance; CPMP/ICH/367/96—Note for guidance specifications: test procedures and acceptance criteria for new drug substances and new drug products: chemical substances; Date for coming into operation: May 2000).
The need of the above-mentioned standardization has further been strengthened just in the field of the rifamycin antibiotics from Henwood S. Q., de Villiers M. M., Liebenberg W. and Lotter A. P., Drug Development and Industrial Pharmacy, 26 (4), 403-408, (2000), who have ascertained that different production batches of the rifampicin (INN) made from different manufacturers differ among them because they show different polymorphous characteristics, and as a consequence they show different profiles of dissolution together with consequent alteration of the respective pharmacological properties.
By applying the processes of crystallization and drying generically disclosed in the previous patents IT 1154655 and EP 0161534 it has been found that under some experimental conditions the poorly crystalline form of the rifaximin is obtained while under other experimental conditions the other crystalline polymorphous forms of the rifaximin are obtained. Moreover it has been found that some parameters, absolutely not disclosed in the above-mentioned patents, like for instance the conditions of preservation and the relative humidity of the ambient, have the surprising effect to determine the form of the polymorph.
The polymorphous forms of the rifaximin object of the present patent application were never seen or hypothesized while thinking that a sole homogeneous product would always have been obtained whichever method would have been chosen within the range of the described conditions, irrespective of the conditions used for crystallizing, drying and preserving.
It has now been found that the formation of the δ and ε forms depends on the presence of water within the crystallization solvent, on the temperature at which the product is crystallized and on the amount of water present into the product at the end of the drying phase.
The form δ and the form ε of the rifaximin have then been synthesized and they are the object of the invention.
In particular the form δ is characterized by the residual content of water in the dried solid material in the range from 2.5% and 6% (w/w), more preferably from 3% and 4.5%, while the form ε is the result of a polymorphic transition under controlled temperature moving from the form δ.
These results have a remarkable importance as they determine the conditions of industrial manufacturing of some steps of working which could not be considered critical for the determination of the polymorphism of a product, like for instance the maintaining to a crystallized product a quantity of water in a stringent range of values, or the process of drying the final product, in which a form, namely form δ, has to be obtained prior to continuing the drying to obtain the form δ, or the conditions of preservation of the end product, or the characteristics of the container in which the product is preserved.
Rifaximin exerts its broad antibacterial activity in the gastrointestinal tract against localized gastrointestinal bacteria that cause infectious diarrhea including anaerobic strains. It has been reported that rifaximin is characterized by a negligible systemic absorption, due to its chemical and physical characteristics (Descombe J. J. et al. Pharmacokinetic study of rifaximin after oral administration in healthy volunteers. Int J Clin. Pharmacol. Res., 14 (2), 51-56, (1994))
Now we have found that it is possible on the basis of the two identified polymorphic forms of rifaximin to modulate its level of systemic adsorption, and this is part of the present invention, by administering distinct polymorphous forms of rifaximin, namely rifaximin δ and rifaximin ε. It is possible to have a difference in the adsorption of almost 100 folds in the range from 0.001 to 0.3 μg/ml in blood.
The evidenced difference in the bioavailability is important because it can differentiate the pharmacological and toxicological behavior of the two polymorphous of rifaximins δ and ε.
As a matter of fact, rifaximin ε is negligibly absorbed through the oral route while rifaximin δ shows a mild absorption.
Rifaximin ε is practically not absorbed, might act only through a topical action, including the case of the gastro-intestinal tract, with the advantage of very low toxicity.
On the other way, rifaximin δ, which is mildly absorbed, can find an advantageous use against systemic microorganisms, able to hide themselves and to partially elude the action of the topic antibiotics.
In respect of possible adverse events coupled to the therapeutic use of rifaximin of particular relevance is the induction of bacterial resistance to the antibiotics. Generally speaking, it is always possible in the therapeutic practice with antibiotics to induce bacterial resistance to the same or to other antibiotic through selection of resistant strains.
In case of rifaximin, this aspect is particularly relevant, since rifaximin belongs to the rifamycin family, a member of which, the rifampicin, is largely used in tuberculosis therapy. The current short course treatment of tuberculosis is a combination therapy involving four active pharmaceutical ingredients: rifampicin, isoniazid, ethambutol and pyrazinamide and among them rifampicin plays a pivotal role. Therefore, any drug which jeopardized the efficacy of the therapy by selecting for resistance to rifampicin would be harmful. (Kremer L. et al. “Re-emergence of tuberculosis: strategies and treatment”, Expert Opin. Investig. Drugs, 11 (2), 153-157, (2002)).
In principle, looking at the structural similarity between rifaximin and rifampicin, it might be possible by using rifaximin to select resistant strains of M. tuberculosis and to induce cross-resistance to rifampicin. In order to avoid this negative event it is crucial to have a control of quantity of rifaximin systemically absorbed.
Under this point of view, the difference found in the systemic absorption of the δ and ε forms of the rifaximin is significant, since also at sub-inhibitory concentration of rifaximin, such as in the range of from 0.1 to 1 μg/ml, selection of resistant mutants has been demonstrated to be possible (Marchese A. et al. In vitro activity of rifaximin, metronidazole and vancomycin against Clostridium difficile and the rate of selection of spontaneously resistant mutants against representative anaerobic and aerobic bacteria, including ammonia-producing species. Chemotherapy, 46(4), 253-266, (2000)).
According to what above said, the importance of the present invention, which has led to the knowledge of the existence of the above mentioned rifaximin polymorphous forms and to various industrial routes for manufacturing pure single forms having different pharmacological properties, is clearly strengthened.
The above-mentioned δ and ε forms can be advantageously used as pure and homogeneous products in the manufacture of medicinal preparations containing rifaximin.
As already said, the process for manufacturing rifaximin from rifamycin O disclosed and claimed in EP 0161534 is deficient from the point of view of the purification and identification of the product obtained; it shows some limits also from the synthetic point of view as regards, for instance, the very long reaction times, from 16 to 72 hours, very little suitable for an industrial use and moreover because it does not provide for the in situ reduction of the rifaximin oxidized that may be formed within the reaction mixture.
Therefore, a further object of the present invention is an improved process for the industrial manufacturing of the δ and ε forms of the rifaximin, herein claimed as products and usable as defined and homogeneous active principles in the manufacture of the medicinal preparations containing such active principle.